tests: move Python 3.5 check higher in file
Per code review comment on the changeset that introduced the Python 3.5+
checks.
Differential Revision: https://phab.mercurial-scm.org/D12259
py2: drop some more from __future__ statements
These are no longer needed after dropping support for Python 2.
Differential Revision: https://phab.mercurial-scm.org/D12258
tests: delete some no-py3 blocks
And drop some nearby avoidable py3 checks in close proximity while
we are here.
Differential Revision: https://phab.mercurial-scm.org/D12257